The disclosure relates to the field of radiation scanning and/or radiation imaging. It finds particular application with computed-tomography (CT) scanners where one or more calibration tables are utilized to correct measurements acquired during an examination of a subject (e.g., person, luggage, etc.). It also relates to other radiation systems where it is desirable to correct measurements acquired during an examination to account for errors due to, among other things, manufacturing defects, electronic noise, and/or degradation of a detector array configured to acquire the measurements, for example.
Radiation systems (e.g., also referred to as imaging systems, radiation imaging systems, radiation scanning systems, and/or the like) such as computed tomography (CT) systems, diffraction CT, single-photon emission computed tomography (SPECT) systems, projection systems, and/or line systems, for example, are used to provide information pertaining to interior aspects of a subject. Generally, the subject is exposed to radiation comprising photons (e.g., such as x-ray photons, gamma ray photons, etc.) to measure attenuation by the subject (e.g., which may be indicative of the density of the subject and/or aspects thereof). In some embodiments, an image(s) is formed based upon the radiation absorbed and/or attenuated by interior aspects of the subject, or rather an amount of photons that is able to pass through the subject. Generally, highly dense aspects of a subject absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, may be apparent when surrounded by less dense aspects, such as muscle or clothing.
To reconstruct an image from measurements acquired during the examination and/or to perform other processes using the measurements (e.g., such as automated threat analysis processes), it is desirable for the measurements to accurately reflect the amount of radiation detected and to reduce (e.g., to a minimum) errors in the measurements (e.g., caused by manufacturing defects in the detector array material and/or readout electronics, degradation of the detector array material and/or readout electronics over time, etc.). Accordingly, a set of calibration procedures may be periodically or intermittently performed to compute correction factors that adjust the measurements acquired from the detector array to correct for the errors (e.g., to reduce the contribution of the errors to the overall measurements). For example, a set of one or more calibration procedures may be performed daily, weekly, and/or at other scheduled times to compute such correction factors.
One such calibration procedure that is commonly performed is an air scan. During an air scan, the detector array is substantially uniformly exposed to radiation. Typically, this is achieved by removing objects, such as a gurney for supporting subjects under examination, from a field of view to provide a clear line-of-sight from a radiation source to the detector array (e.g., such that radiation experiences little to no attenuation during the air scan). Accordingly, differences in measurements between detector cells of the detector array may be attributed to error, and a gain correction for respective detector cells may be identified that corrects the measurements from the corresponding detector cell (e.g., to reduce differences in the measurements between detector cells when an air scan is performed). The gain corrections are typically stored in an air table, also referred to as an air calibration table, and applied to measurements acquired during an examination of a subject to correct the measurements (e.g., such that variations in the corrected measurements reflect variations due to attenuation by the subject, and not due to errors).